3.4.2. Processing of Structural Steel 41Cr4

The objects of study during the tests were the wear of five-flute end mills made of 6WH10F hard alloy (Figure 2) and the surface roughness of workpieces in the form of bars made of structural steel 41Cr4. As can be seen from the experimental data presented in Figure 13, in the process of machining structural steel, rather intense wear of the uncoated end mills occurs; the operating time to failure (until the wear limit value of 0.4 mm is reached) is slightly more than 30 min. Simultaneously, the application of a single-layer DLC to the cutter has little effect on the wear rate of the tool and only slightly increases its tool life (no more than 4 min). Optical images (Figure 14), showing the general view of the worn chamfer on the flank surface of the cutters after 17.5 min of operation, well illustrate the above well—the coating application practically does not slow down the development of wear processes.

**Figure 13.** Dependences of the wear chamfer size on the flank surface of the end mills on the operating time when machining 41Cr4 structural steel with end mills with different coatings.

**Figure 14.** The size of the flank wear chamfer of end mills after 17.5 min of work when processing structural steel 41Cr4 with end mills; (**a**) uncoated, 182.21 μm; and (**b**) with a single-layer DLC, 151.90 μm.

Analysis of the development of wear in end mills after applying the sublayer (CrAlSi)N in combination with the formation of a DLC shows a noticeable effect of a two-layer coating on the tool wear rate when milling 41Cr4 structural steel. The tool life of end mills with (CrAlSi)N/DLC was 64 min, which is 2.0 times higher than the corresponding indicator of the uncoated tool (32 min) and 1.8 times higher than the tool with a single-layer DLC (35 min).

The dependences of the change in the surface roughness of the 41Cr4 steel workpiece on the operating time of the tool with different coatings shown in Figure 15 allow us to reveal the following regularity. From the very beginning of the cutting process, up to 25 min of work, the quantitative value of the roughness formed on the workpieces has a comparable value for all investigated mills and is in the range of 2.5–3.0 μm. After 25 min of operation, uncoated end mills show a noticeable deterioration in the quality of the machined surface and its sharp increase; at the time of tool failure (32 min), the roughness of the workpiece is more than 6 μm. For mills with a single-layer DLC, a significant increase in the roughness of the processed workpiece is also observed, and by the time of tool failure (35 min), it is about 5 μm. The (CrAlSi)N/DLC makes a slightly more significant contribution to the reduction of roughness. Thanks to this coating, the surface roughness gradually increases over time, reaching the value of 8.75 μm at the time of failure (64 min).

**Figure 15.** Dependence of the roughness of the treated workpiece surface of structural steel 41Cr4 on the operating time of end mills with different coatings.
